The control of blood flow in the brain is a truly dynamic process that is continuously being adjusted to meet the demands of local neuronal activity. Much of this control occurs at the level of the resistance blood vessels that requires synchronized changes in blood vessel diameter. In order to meet this need, endothelial cells and smooth muscle cells must be able to communicate with one another. One way in which this cell-to-cell communication is mediated is through structures in the plasma membrane, called gap junctions. These gap junctions act as bridges between two cells, thereby allowing the passage of ions and small molecules. They can form endothelial cell-endothelial cell, smooth muscle cell-smooth muscle cell connections or endothelial cell-smooth muscle cell connections (myoendothelial). Another way in which the cells of the vascular wall communicate is through the production of vasoactive substances including nitric oxide, prostacyclin and another as yet unidentified entity called endothelium-derived hyperpolarizing factor (EDHF). My laboratory has shown that EDHF-mediated dilations in middle cerebral arteries are attenuated in females compared to males, an effect attributed to estrogen. Preliminary studies indicate that gap junctions mediate the EDHF signal transduction pathway in cerebral arteries. Interestingly, gap junctions can be modulated by estrogen. In the current proposal, three specific hypotheses will be tested. Hypothesis 1 is that gap junctions are located in the male rat MCA. Hypothesis 2 is that myoendothelial gap junctions (MGJ) are involved in the EDHF response. Hypothesis 3 is that the attenuation of the EDHF response in females is attributed to MGJ uncoupling by estrogen. These hypotheses will be tested by measuring vascular function (diameter and membrane potential) and by using a battery of molecular techniques including RT-PCR (mRNA), Western analysis (protein expression), immunofluorescence coupled with electron microscopy (anatomical evidence of MGJ), and electrical coupling (functional evidence of MGJ). This work will have a tremendous impact on our understanding of the participation of intercellular communication in controlling vascular tone and thus blood flow within the cerebral circulation.